Optical Communication Systems are desired because of the wide bandwidths available for the information signal channels. However, the available wide bandwidths for the signal content also permits noise to be present over the same wide bandwidths so that both the desired signals and undesired noise is amplified by the optical system amplifiers. In addition, rare earth-doped fiber amplifiers (such as Erbium or Erbium-Ytterbium doped), while useful as broad band optical amplifiers have a significant drawback in that the gain is not uniform over the optical wavelength. This becomes a particularly significant problem in wavelength division multiplexed (WDM) Systems, where multiple wavelengths are to be amplified simultaneously.
An approach which introduces excessive loss has been the use of complex passive filters designed to "flatten" the gain versus wavelength curve of broad band optical amplifiers. Another approach discussed in U.S. Pat. No. 5,283,686 is to combine an Erbium fiber optic amplifier with a narrow bandwidth optical filter. The optical filter is formed of an in-fiber Bragg grating induced in the fiber by photo-refractive effects so that the grating reflects a vary narrow band of wavelengths constituting the desired signal. This approach, however, while improving the gain versus wavelength characteristics selectively at different frequencies, does not effectively flatten the curve response or provide a precise control for multiple signals across a broad band of wavelengths, such as would be desirous in a WDM System.